Recently, a thermally resistant polymer composition has been widely used in automotive parts, electric/electronic products, machine components, beverage containers, fibers, films, tire cords, and the like. Examples of such polymer may include aliphatic polyamide polymers, such as Nylon 66 and Nylon 6, aromatic polyamide polymers (high heat resistant nylon resins) such as Nylon 6T, Nylon 9T, Nylon 10T, and Nylon 12T, polyester resins, such as polyethylene terephthalate (PET) reins, and polycarbonate (PC) resins.
Typically, in order to enhance heat resistance and impact resistance of a polymer, it is necessary to increase inherent viscosity (IV) of the polymer. For example, as a method for obtaining a polymer having an inherent viscosity of about 0.5 dL/g or higher, condensation polymerization of molten resins, which is referred to as a melting process, can be performed.
However, in such a melting process, a product is subjected to high shear due to high viscosity of a polymer (particularly, crystalline polymer) during condensation polymerization and transfer thereof, which can cause breakdown of the product. To solve this problem, the polymer must be heated over its melting point during the melting process. Thus, particularly, condensation polymerization of high heat resistant resins having high melting point requires enormous energy and is thus uneconomical. In addition, since the polymer is likely to carbonize in a long-term operation, a large amount of carbonized contaminants can be contained in final products, and the products can suffer from discoloration and thus can be unsuitable for applications requiring high whiteness.
To overcome these problems, solid-state polymerization (SSP) is commonly used. Typical solid-state polymerization includes a process in which amorphous polymer chips, prepolymers, and the like are placed in a solid-state polymerization reactor and heated for several to tens of hours while supplying an inert gas into the reactor in a circulating manner.
As an inert gas, nitrogen heated to a temperature higher than or equal to glass transition temperature of the polymer and less than the melting point of the polymer, for example, from about 130° C. to about 250° C., is mainly used. When an active gas such as oxygen is present in a polymerization system, some products can suffer from discoloration such as serious yellowing or browning during polymerization at high temperature. For this reason, an inert gas is used in solid-state polymerization. In other words, by circulating an inert gas through a reactor, inflow of an active gas can be minimized, and reaction by-products such as water, aldehydes, glycol, and phenol can be discharged together with the inert gas. However, when the by-products are recirculated into the solid-state polymerization reactor, purity of the circulated inert gas can be gradually reduced, thereby causing discoloration of the polymer, reduction in reaction rate, or even reverse polymerization due to high concentration of the by-products. Thus, the by-products must be removed from the inert gas prior to reintroduction of the inert gas. As such, typical solid-state polymerization requires lots of energy and costs to remove by-products such as water from an inert gas flow and maintain purity of the inert gas, and is thus uneconomical.
Examples of a typical batch type solid-state polymerization apparatus include a stationary apparatus in which reaction proceeds while stirring with a rotary vane mounted on a top side of a vertical reactor, and a tumbler type apparatus in which prepolymers are introduced into a reactor, both upper and lower portions of which are conical, and the reactor is sealed, followed by performing reaction while rotating the entire body of the reactor under a vacuum (Japanese Patent Laid-open Publication No. 2001-270940A, and the like).
In addition, examples of a typical continuous solid-state polymerization apparatus include a hopper type apparatus (WO1998-023666, and the like) and a horizontal circular reactor type apparatus (Japanese Patent Laid-open Publication No. 10-87821, and the like). The hopper type apparatus includes a vertical reactor having a cylindrical upper portion and an inverse conical bottom portion, wherein prepolymers are introduced to the upper portion while introducing a heated inert gas in the vicinity of the inverse conical bottom portion, such that the final products (polymers) are discharged to the bottom and the inert gas containing impurities generated during reaction are discharged to the upper portion. The horizontal circular reactor type apparatus includes a transverse reactor having a screw or disk type stirring vane therein, wherein mixing is performed using the stirring vane while simultaneously introducing prepolymers and a heated inert gas through an inlet, such that products (polymers) are discharged towards an outlet opposite the inlet and the inert gas containing impurities is discharged upwards near the outlet of reactor.
A batch type reaction apparatus can maintain a thermal history of prepolymers in a relatively constant manner as compared with a continuous reaction apparatus, thereby obtaining a uniform inherent viscosity (IV), but has disadvantages of low output per batch lot, long cycle time, and huge energy loss due to continuous repetition of heating and cooling processes for reaction, all of which lead to increase in product costs.
A continuous reaction apparatus is relatively steady as compared with a batch type reaction system, has high output per lot due to short cycle time, allows mass production even with a small sized apparatus, and can thus provide low investment costs and low energy loss thereby reducing product costs. However, in the continuous reaction apparatus, in molding into end products, a cycle time required for molding into one product is increased due to a broader molecular weight distribution than the batch type reaction apparatus, which can lead to reduction in productivity. In addition, in the case of a transverse reactor having a fixed reactor body and a rotatable stirring vane, in solid-state polymerization, an empty space must be created between an end tip of the stirring vane and an inner wall of the reactor in order to avoid problems of thermal expansion of the apparatus. However, since a stagnation zone through which few or no polymer flows can be created in that space, carbonized contaminants or the like are likely to be generated, causing deterioration in product quality, and prepolymers are likely to be crushed, causing changes in grain size. In addition, the continuous reaction apparatus has difficulty in continuous supply/discharge under a vacuum, and thus commonly employs an inert gas. However, heating and cooling of the gas must be repeated, and a separate purification process is required to recover a pure inert gas, from which by-products are removed. Thus, the continuous reaction apparatus exhibits poor energy efficiency as compared with the rotatable batch type solid-state polymerization apparatus.
Therefore, there is a need for a continuous solid-state polymerization apparatus (reactor) which can prevent formation of a prepolymer stagnation zone and can perform reaction under a vacuum.